Rain Garden Project
Making Your Own Rain Garden
Planting a rain garden in your own yard is a great way to conserve the local water resources, and also have a positive effect on the environment all along the Ohio River, the Mississippi River and even into the Gulf of Mexico.
There are many references available that describe how to design and construct a rain garden in your own yard. Here is a sample rain garden for your home, designed by Mesker Park Zoo & Botanic Garden. The plants selected are suitable for our region’s heavier soils. For more information or a detailed review of the process you can download a rain garden manual from Wisconsin’s DNR. Here is a quick summary of the important aspects of rain garden construction:
The location you select within your yard will be a big part of the success of your rain garden. Make sure your rain garden is at least 10 feet away from any buildings, because you do not want to increase infiltration in a location where it might damage foundations or flood basements. You also do not want the rain garden directly over septic tanks or sewer lines, or near large trees, because there is already enough going on underground in these locations.
You want to make sure that the soil underneath your rain garden has a quick enough infiltration to handle the additional storm water that you are going to add. Avoid areas of the yard that the soil has been compacted by people walking or where vehicles have been parked. In order to determine if the infiltration rate is fast enough in a prospective location, dig a hole about one and a half feet deep and half a foot diameter. Fill the hole completely with water and wait for it to drain. If it drains in less than 48 to 72 hours, the infiltration of that location should be sufficient to support a rain garden. You don’t want your rain garden to pool any longer than this, or mosquitoes might breed in the standing water.
Design and Layout
You want runoff to flow into the garden from inclined and impermeable surfaces. You can install a drain pipe to the bottom of one or more gutters to direct rain from your roof into the rain garden, and you can also direct rain from hills or other slopes into your garden. Re-directing runoff from existing driveways or other paved surfaces might require a little more work, but can also be done.
The bottom surface of the rain garden should be slightly below the ground, and it should be level. If your rain garden is on a sloped hill, you can use the dirt from the higher side of the garden to fill in the lower side to make a level surface. If the hill slopes too much, you might need to purchase additional dirt or use dirt from elsewhere in your yard. Putting rain gardens on a large sloped hill makes the construction more complicated and is not the best choice of location for a rain garden.
The berms are the retaining walls of the rain garden that hold the water. The berms should be approximately half a foot across of firmly packed dirt and surround the downhill side. The berms can usually be made from dirt dug out of the center of the rain garden, and they should be planted with grass or covered with mulch to prevent erosion. If the rain garden is observed to be overflowing after heavy rains, a trench should be dug across the top of a berm to allow the water to escape. This prevents erosion of the berms.
The depth of a rain garden is determined by the height of the berms, and a typical rain garden should be from four to eight inches deep from the top of the berm to the bottom surface of the garden. (Remember when making the berms that adding mulch will raise the bottom of the garden). The square footage of your rain garden is determined as a fraction of the drainage area. The drainage area includes all the areas that provide storm runoff to the rain garden. The surfaces that make up the drainage area can either be impervious surfaces, such as a roof or a driveway, or permeable surfaces, such as sloped grassy areas.
The simplest size calculation is that your rain garden should be equal to 90% of the area of impervious surfaces from the drainage area plus 25% of the area of permeable surfaces from the drainage area. (Depending on the type of soil that makes up the permeable surfaces, that 25% can be raised or lowered. Clay soils are more impermeable than sandy soil; if the soil in your yard is more clay-like, you could use 40% instead of 25%, and if your soil is sandier you could use 20%). If you are directing a gutter downspout into your rain garden, you need to estimate how much area of the roof is serviced by that downspout.
So to determine the recommended square footage of your rain garden, measure the area of impervious surfaces in the drainage area and multiply by 0.9 to get the first size, and then measure or estimate the area of permeable surfaces in the drainage area and multiply by 0.25 to get the second size. Add these two sizes together to get the total recommended size for your rain garden. However, a rain garden that is smaller than this recommended area still will help to reduce runoff. Don’t be afraid to adjust the actual size of your rain garden down from this recommended size. However, keep an eye on the integrity of your berms if you undersize your rain garden too much; you
Plants for your rain garden should be local varieties that do not require too much work. Consideration should also be made for the flowering season of each plant; staggering the flowering season will provide you with a kaleidoscope effect as the flowers come and go during the year. You can also take into account what sort of habitat the different plant species create for insects or birds that will be attracted.
A list of plants appropriate for rain gardens in central and southern Indiana has been compiled by the Monroe County Highway Department in Bloomington.
Planting plugs, seedlings, or container plants are the easiest for starting a rain garden, planting seeds is not recommended. The rain garden should be mulched initially, and you make sure that the plants are watered regularly until the plugs are well established.
Weeding of your rain garden will be necessary in the first couple of years, but after that the plants in the garden should be able to fend for themselves. Depending on the weather, you should not have to water your rain garden.